This invention relates to a process for recovering caustic alkali from spent alkali liquor, and more particularly to an improved process for recovering from spent alkali liquor caustic alkali of high purity and concentration by hydrolyzing at a temperature above 110.degree. C. a roasted product of the spent alkali liquor and iron oxide.
The term "spent alkali liquor" as herein used means waste liquors which occur in various chemical processes in which an organic substance is treated with alkali, for instance, a waste liquor discharged from a process where an organic acidic substance is treated with alkali, including the alkali-containing waste liquors discharged from pulp cooking and bleaching processes using caustic alkali.
In recovering caustic alkali from spent alkali liquors which are discharged from various chemical processes, it is known to burn and roast a spent alkali liquor together with iron oxide and to hydrolyze the roasted product at a temperature below 100.degree. C. and under normal pressure.
For example, Japanese Patent Application No. 49-21631 describes a process in which a spent alkali liquor discharged from a fibrous material cooking or bleaching operation by the use of caustic alkali is burned and melted together with iron oxide at a temperature of 700.degree.-900.degree. C. and the resulting melt is dropped into water and dissolved at a temperature of 50.degree.-100.degree. C. to obtain an aqueous solution of caustic soda.
In Japanese Patent Application No. 51-154920, waste organic acidic substances which are produced in various chemical processes, such as spent phenol and carboxylic acids, are neutralized by reaction with a caustic alkali solution, followed by the steps of removing an organic alkali salt solution by extraction and/or absorption, roasting the organic alkali salt solution together with ferric oxide in an oxidative atmosphere over 800.degree. C. to obtain ashes and, after or while grinding the ashes, leaching out caustic alkali with water for separation from the sediment which mainly consists of ferric oxide.
The caustic alkali recovery processes described in the above-mentioned two Japanese Patent Applications are based on the following Reactions (1)-(3): EQU M.sub.2 CO.sub.3 +Fe.sub.2 O.sub.3 =M.sub.2 Fe.sub.2 O.sub.4 +CO.sub.2 ( 1) EQU m.sub.2 o+fe.sub.2 O.sub.3 =M.sub.2 Fe.sub.2 O.sub.4 ( 2) EQU m.sub.2 fe.sub.2 O.sub.4 +H.sub.2 O=2MOH+Fe.sub.2 O.sub.3 ( 3)
wherein M represents an alkali metal.
Upon combustion, the spent alkali liquor with organic contents as mentioned above is converted into alkali carbonate and/or alkali oxide, which simultaneously undergo Reactions (1) and (2), respectively, to result in alkali ferrate. According to Reaction (3), the alkali ferrate is hydrolyzed into caustic alkali and ferric oxide, normally at a temperature below 100.degree. C.
However, it has been found that the velocity of hydrolysis in the above-mentioned known processes is unexpectedly low especially where the produced alkali concentration is relatively high, and the yield of hydrolysis is at most about 60-70% at a temperature below 100.degree. C. when the alkali contentration is over 10%.
In the above-mentioned known processes, the loss of non-recovered caustic alkali, which forms in the conventional hydrolysis condition, can be prevented by recirculating the alkali ferrate residues which have not undergone the hydrolysis. However, recirculation of a large quantity of residual alkali ferrate requires apparatus of large scale and is disadvantageous from the standpoint of energy economy. It will thus be obvious that the rate of hydrolysis should be as high as possible.
Generally, the yield of hydrolysis is further lowered when recovering an aqueous solution of caustic alkali of such a high concentration as to have any practical value, about 15 wt% or more, coupled with another disadvantage in that the recovered solution contains dissolved iron in a high concentration. The presence of a large quantity of dissolved iron in the recovered aqueous alkali solution limits the use of the solution or otherwise will bring about undesired reactions or contamination in use. Therefore, it is advantageous to reduce the dissolved iron content in the recovered aqueous alkali solution to a minimum.